1. Field of the Invention
The invention relates to ceramic clearance control (abradable) materials for high temperature applications, and more particularly to a ceramic abradable material that is doped with alumina, whereby the addition of alumina imparts a significant improvement in erosion resistance to the abradable compared to other commercially available ceramic abradables.
2. Description of the Related Art
Gas turbine engines are used in a variety of applications, including aircraft engines and various other industrial applications where operating temperatures may be in the range of 1000-1200 degrees Celsius and higher. A gas turbine engine is an internal-combustion engine that is typically made up of an air compressor, combustion chamber, and turbine wheel that is turned by the expanding products of combustion. It essentially converts the chemical energy of a liquid fuel into mechanical energy by internal combustion. The operational efficiency of gas turbine engines increases with the rise in operational temperatures which can place components under increased stress as well as subject those components to thermal expansion.
In order to maximize efficiency in the operation of turbine engines, it is desirable to minimize the clearances between the turbine blade tips and the outer casing or shroud. Too large a clearance will result in poor fuel efficiency in order to provide the desired operational speeds, while too small of a clearance risks contact between the turbine blade tips and the outer casing which can result in component failure. To solve this problem, conventional gas turbine engines include an abradable coating or seal that is applied to the surface of the outer casing and/or seal. These abradable coatings or seals can be cut by the rotating turbine blades without damaging the turbine blades. In addition, the abradable coatings or seals must remain resistant to erosion that is caused by small solid particles and high gas flow rates present in the turbine. Erosion refers to wear caused by a stream of small particles and is analogous to abrasive wear.
The abradable coating or seal must also have good abradability, spall resistance, low gas permeability, a smooth surface, good aging properties and long term oxidation resistance.
Conventional gas turbine engines may utilize porous ceramic abradable materials, such as yttria stabilized zirconia (YSZ) (8 wt. % Y2O3, bal. ZrO2). Although such YSZ materials are suited for use as abradable materials, they are relatively friable and susceptible to erosive wear effects. Thus, there is a continued need for superior abradables with acceptable abradability performance and erosion resistance so that tolerances can be optimized even further to achieve improved efficiency and performance.